This is what I got from the Microsoft Web Site
Simple Licensing Model
Products available to license via the Services Provider License Agreement (SPLA) are priced on a monthly basis. The Microsoft software products included in the program are available for licensing through two models:
1. Per User (Subscriber Access License)
2. Per Processor (CPU License)
Subscriber Access License (SAL)
In the SAL mode, a SAL is required for each unique individual who is authorized to use the software in any given month. Not all products are available to be licensed in the SAL Mode. Benefits of the Per Subscriber model include:
* Subscribers can access any number of servers from any
number of devices
* Service providers can "scale out" by deploying as many
servers as they need
* Service providers have minimal start-up costs since they
only pay based upon the number of subscribers they have each
Processor License (CPU)
When licensing via the CPU licenses, an unlimited number of end users can use the software running on a single CPU for each CPU license acquired. Some products are not available to be licensed in the Processor mode. Benefits of the Per CPU Model include:
* CPU licenses are easier to monitor and count, which lowers
the service provider's administrative costs.
* CPU licenses replace Internet Connector licenses,
eliminating the need to distinguish between Internet,
intranet, and extranet users.
* CPU licenses are economical across a variety of business
scenarios where the utilization of the software may vary.
For products that can be licensed in either Per Processor/CPU or Per User/Subscriber mode, the service provider can switch licensing mode from one month to another.